Continuous casting process



April 2, 1968 C.J.OPEL ETAL CONTINUOUS CASTING PROCESS 4 Sheets-Sheet lFiled Sept. 28, 1965 5 u, i 5 m f MQW M Nf a E mf WA i MA ff w S am mApril 2, 1968 C.J.OPE| ETAL CNTINUOUS CASTING PROCESS 4 Sheets-Sheet 2Filed Sept. 28, 1965 April 2, 1968 c. J. OPEL ETAL CONTINUOUS CASTINGPROCESS 4 Sheets-Sheet 4 Filed Sept. 28, 1965 mam i WW U 3,376,37lPatented Apr. 2, i968 3,376,371 CONTINUUUS CASTiNG PROCESS Charles J.Opel, Huntington Beach, and Paul H. Bottoms, Hollywood, Calif.,assignors to Swedlow, Inc., Garden Grove, Calif., a corporation ofCalifornia Filed Sept. 28, 1965, Ser. No. 490,945 10 Claims. (Cl.264-216) The present invention relates to the continuous casting ofpolymeric compositions. There has been substantial interest in castingpolymeric compositions for many years and many processes have beenproposed and developed for such casting. Much of this development workhas concerned the casting of acrylic and other polymerizable unsaturatedorganic materials. While the present invention is particularlyapplicable to the polymerization and casting of unsaturated compoundssuch as monomers, it is understood that it is applicable to a wide rangeof polymerizable materials such as pre-polymers and the like, all ofwhich materials will be referred to in the specication and claims forconvenience as monomers.

In general, the casting processes which have been previously developedcomprise subjecting the polymerizable compositions to heat and/or lightwith or without the application of pressure. A highly successful andwidely used process comprises casting liquid monomer between a pair ofrigid sheets which sheets are provi-ded with a peripheral gasket ofexible material to compensate for the shrinkage which occurs duringpolymerization, the assembly being immersed in a hot water bath or hotair oven for the length of time required to carry out the thermalpolymerization. The hot water bath is, of course, the source of bothheat and pressure. This process has come to be known as cell casting.Cell casting is, as is apparent from the foregoing description, a batchprocess and, although widely used, is subject to all of the deficienciesof any batch process, viz, a substantial amount of down time betweencasting operations, size limitations, quality control diiculties due toinadvertent changes in pressure, temperature and composition, etc.

Thus, there has long been substantial interest in the development of acontinuous casting process, but, prior to the present invention, acommercially practicable continuous casting process has not beendeveloped. In this regard, it should be pointed out that the diicultiesinvolved in continuous casting become increasingly greater as thethickness of the cast sheet is increased.

One proposal with regard to continuous casting is that disclosed inUnited States Patent No. 2,500,728 wherein it is suggested that a pairof endless belts arranged such that they are vertically spaced from eachother with the lower run of the upper belt and the upper run of thelower belt substantially parallel to each other might be used as acontinuous casting apparatus. While this proposal appeared to havesubstantial possibilities from a theoretical standpoint, it was foundthat the apparatus specifically disclosed in Patent No. 2,500,728 wasnot suitable for commercial operation because of its short length andthe extremely slow speed of travel required to bring the necessarypolymerization -to completion such that a final continuous, rigid sheetwould be formed. It was then, of course, proposed that an apparatus ofthe type disclosed in Patent No. 2,500,728 be produced in which thebelts were of substantially greater length. However, when the apparatuswas designed such that it would be suitable for commercial operationfrom a size standpoint, e.g., five feet wide and 100 feet long, it wasfound that the spacing between the belts could not be maintained if usedin the manner described in Patent No. 2,500,728. This, of course,resulted in loss of control over the thickness of the final product andcaused non-uniformity in this thickness. This problem was found to beparticularly acute when stainless steel belts, which are preferred, wereused. In general, this inability to maintain proper spacing between themoving belts results from the lack of support for the lower run of theupper belt which tends to bow downwardly toward the upper run of thelower belt.

The present invention obviates the foregoing and other diiiculties whichhave been inherent in the continuous casting processes previouslyproposed and makes possible, for the rst time, continuous casting ofacrylic and other polymerizable monomers in a manner which is suitablefor commercial use and which produces a product having acceptablecharacteristics.

It is a primary object of the present invention to provide a process forthe continuous casting of polymerizable organic monomers wherein theliquid organic monomer is cast between a pair of moving endless beltswhich are so arranged and constructed that the spacing between them iscontrolled and maintained at a desired predetermined distance.

It is another object of the present invention to provide a process forcontinuous casting of acrylic and other polymerizable monomerscomprising a pair of moving endless belts which are inclined in such amanner that the pressure of the materials between the belts issuflicient to maintain the spacing between them at a desiredpredetermined distance.

It is a further object of this invention to provide a process forcontinuous casting of acrylic and other polymerizable monomers which issuitable for commercial use.

`Other objects and advantages of the present invention, it is believed,will be apparent from the following detailed description of specificembodiments thereof.

Broadly, the apparatus used in the method of the present inventioncomprises a pair of endless belts mounted on rollers in such a mannerthat the lower run of the upper belt is substantially parallel to theupper run of the lower belt for a substantial portion of their lengths.These belts are arranged such that the lower run of the upper belt andthe upper run of the lower belt are inclined at an angle to thehorizontal. In operation, each of the belts is caused to move `aroundits supporting rollers such that ythe lower run of the upper belt andthe upper run of the lower belt move in the same direction, and suchthat there is substantially no relative movement between the lower runof the upper ibelt and the upper run of the lower belt. Liquidpolymerizable monomer is fed to the intake ends of these belts and ispassed through at least one heating station. In the preferred embodimentof the process of the present invention, a liquid acrylic monomercomposition is fed into l'the casting apparatus and is lirst heated -toa sufficient extent to cause the monomer to polymerize (or cure) and isthen subjected to a further heating step to postcure the polymerizedcomposition. The initial heating step may be performed by heating withhot air, by causing the moving belts to pass through a zone which isllooded with hot water or, preferably, may be accomplished Iby sprayinghot Water on the external surface of the moving belts. In any event, thecritical requirement of the apparatus and process of the presentinvention is that the angle of inclination to the hori zontal of thebelts be such that a pressure head of the liquid monomer is developedwhich is suicient to maintain the spacing ybetween the bel-ts and thedesired distance until the liquid has polymerized. As claimed herein,the inclined path of the moving belts may be a str-aight line or, asclaimed in copending application Ser. No. 490,897, led on the same dateas the present application in the name of Hellsund, and assigned to theassignee of the present application, these belts may follow a curvedpath which is defined by the natural curvature of the belts.

Referring now to the drawings, FIGURE la illustrates a portion of anembodiment of the present invention 3 wherein the path defined by themoving belts is the natural curvature of these belts.

FIGURE 1b illustrates the remainder of the apparatus illustrated inFIGURE la.

FIGURE 2 is a vertical cross-section taken on line 2-2 of FIGURE 1awhich illustrates the spray means used as -a source of heating medium.

FIGURE 3 is a vertical crosssection taken on line 3-3 of FIGURE 2 whichillustrates a -side view of the t spray means and moving belts.

FIGURE 4 is a ventical cross-section taken on line 4 4 of FIGURE lb andshows the moving belts passing through the postcuring oven.

FIGURE 5 is a vertical cross-section taken on line 5-5 of FIGURE 4 andillustrates a side view of the moving belt passing through thepostcuring oven.

FIGURE 6 illustrates another embodiment of the present invention whereinthe path of the moving belt is a straight line which is inclined to thehorizontal.

FIGURE 7 is a vertical cross-section taken on line 7 7 of FIGURE 6 andillustrates the water looding heating means which may be used in thepresent invention.

As shown in FIGURE 1a and FIGURE lb, one embodiment of the apparatusused in the method of the present invention comprises an upper belt 11which is positioned above a lower belt 12. Upper belt 11 is supported onrollers 13 and 14 while lower belt 12 is supponted on rollers 15, 16,17. As is seen most clearly `in FIGURE la, lower belt 12 is somewhatlonger than upper belt 11 so as to provide exposed area 18 which servesas a convenient means for feeding liquid monomer between rollers 13 and16. Support posts 19' which are provided with rollers and rollers 21provide additional support for the belts 11 and 12. Roller 14 is mountedon movable support 22 and support 22 is in turn connected to piston 23.Thus, the tension exerted by the rollers on belt 11 may be adjusted byactuating piston 23 so as to move support 22 and roller 14. Roller 15 issimilarly mounted on movable support 24 which support is connected topiston 25 so that the tension exerted by the rollers on belt 12 may alsobe adjusted. Rollers 14 and 17 are driven by motor 26 .through chains 27and 28.

Also illustrated in FIGURES la and lb are the housing 29 for the spraymeans which are used to provide lthe heat for polymerization of themonomer and housing 30 of the postcuring oven. These heating means areillustrated in more detail in FIGURES 2-5.

As shown in FIGUR-ES 2 and 3, housing 29 encloses a series of spraymeans some of which are positioned above the lower run of the upper beltas indicated generally by numeral 31 and some of which are positionedbelow the upper run of the lower belt and are indicated generally bynumeral 32. Preferably, -spray nozzles 33 are so constructed such that asquare pattern is sprayed by each nozzle on belts 11 and 12 in themanner indicated by the broken lines emanating from each nozzle 33. Eachof the nozzles 33 is mounted on one of pipes 34 and the pipes 34 areconnected to conduits 35. Conduits 35 are connected to a source of hotwater or other suitable liquid (not shown). As shown in FIGURE 2,housing 29 is provided with a supporting gusset 36. Gaskets 50 and y51are also provided between belts 11 Aand 12 to prevent leakage of thepolymerizable composition.

As further shown in FIGURES 2 and 3, a series of rollers 37 are providedabove the lower run of belt 11 and a desired inclined path which isgenerally parallel to belt 12. Furthermore, since shrinkage generallyoccurs during polymerization, this shrinkage must be compensated for bycausing belts 11 and 12 to assume a path which converges to the sameextent as the decrease in thickness caused by polymerization shrinkage.Preferably, this may be accomplished by providing spring-loadedmountings for rollers 37, such as those shown in FIGURE 7 for rollers77, or any other suitable means may be used such las adjustablemountings. Rollers 37 and 38 are preferably arranged such that they forma square pattern, i.e., such that each roller is equidistant from itsadjacent rollers in both the lateral and longitudinal direction.

Preferably, housing 29 is constructed in sections, two of which areindicated generally by numerals 39 and 40 in FIGURE 3. Each of thesesections is supported at one end by support member 19 and at the otherend by support member 41 as seen most clearly in FIGURE la. As shown inFIGURE 3, the exit end of each section is provided t with wiper means 42and 43 which substantially prevent the water or other liquid on thesurfaces of belts 11 and 12 from leaving section 39.`

Refering now to FIGURES 4 and 5, We find the postcuring section which issurrounded by housing `30. AsA

shown in FIGURE 5, housing 30 is adjacent to the last section of housing29 and wiper means 44 and `45 are provided to substantially prevent thewater or other liquid on the surfaces of belts 11 and 12 from enteringhousing 1 30. A series of upper rollers 46 and a series of lower rollers47 are also provided in housing 30 to supportfbelts 11 and 12 in muchthe sarne manner as rollers 37 and 38 in housing 29. However, rollers 46and 47 are so arranged i that the belts 11 and 12 assume a convergingpath in order product. Preferably, rollers 46 are spring mounted inorder to compensate yfor the shrinkage which might occur duringpost-curing. If the product shrinks away from the belts,

certain defects including surface irregularities will occur in theproduct. Preferably, rollers 46 are spring mounted in order to urge belt11 toward Ibelt 12 and thereby compensate for the shrinkage. It is alsopreferred to provide rollers 38 and 47 with fixed mountings. Housing 30may might remain on the surface of belts 11 and 12 and may comprise anyconventional means such as spray means, rotating brushes, solventsolution, etc. It is, of course, desirable to remove any deposits whichmay be present on the surfaces of belts 11 and 12 since such depositswould otherwise result in the formation of irregularities on the surfaceof the cast sheet.

Referring now to FIGURES 6 and 7, another embodiment of the presentinvention is illustrated in which endless belts 50 and 51 are arrangedsuch that the path of the lower run of belt 50 and the upper lun of 51is a substantially straight line which is inclined to the horizontal.

As shown in FIGURE 6, belt 50 is mounted on rollers 52 and 53 while belt51 is mounted on rollers 54 and 55. Roller 52 is preferably adjustablymounted as illustrated in FIGURE 6 such that screw 56 may be used toadjust the position of axle 57 in mounting 58 to thereby adjust thetension exerted by rollers 52 and 53 on belt 50. Roller 54 is similarlypreferably `adjustably mounted such that screw 59 `may be used to adjustthe position of axle 60 in mounting 61 to adjust the tension exerted onbelt 51. Rollers 53 and 55 are driven by motor 62 through chains 63 and64.

It is to be understood that the spraying means illustrated in FIGURES 2and 3 may be used with the embodiment of the present inventionillustrated in FIGURE 6. However, for the purpose of completeness, awater flooding means which may be used as an alternative heating meansis illustrated in FIGURES 6 and 7. This water flooding heating meanscomprises an upper water inlet pipe 65 which branches into feeder pipe66 which feeder pipe in turn branches into a pair of delivery pipes 68.As shown in FIGURE 6, delivery pipes 68 deliver hot water to the inletend of the upper portion of the heating means. Similarly, hot water issupplied to the inlet end of the lower portion of the water floodingmeans by pipe 70. Pipe 70 is surrounded by housing 72 which is providedwith front and back walls 73 and 74. A slight clearance is maintainedbetween belt 51 and the upper edges of Walls 73 and 74 such that thewater in housing 72 may overiiow into drainage tank '75 and from tank 75into pipe 76 which conveys the -water to a recirculating pump andheating unit (not shown).

Rollers 77 are provided above the lower run of belt 50 and rollers 78are provided below the upper run of belt 51. As explained with regard torollers 37 and 38, rollers 77 function to support belt 51 while rollers77 maintain belt 51 in the desired position and exert pressure on thematerial being cast. Rollers 77 are illustrated as being spring loadedin FIGURE 7 such that springs 79 urge these rollers toward belt 50.However, rollers 77 may also be provided with a fixed mounting.

A postcuring means which is indicated generally by numeral 80 is alsoillustrated in FIGURE 6. The postcuring means comprises a conventionalforced air heating system. As in the case of rollers 46 and 47, rollers86 and 81 are mounted such that the shrinkage which occurs duringpostcuring is compensated for by causing the belts 50 and 51 to assume aconverging path. Preferably, this is accomplished by providing rollers86 with a spring loaded mounting while providing rollers 81 with a fixedmounting. IRollers 7 S are preferably provided with a fixed mounting asillustrated in FIGURE 7.

In order to prevent the escape of fluid materials froml between belts 50and 51, a pair of gaskets 82 and 83 may be provided. Gaskets 82 and 83may travel at substantially the same speed as belts 50 and 51 and thusprovide a dynamic side seal for the material being cast.

The operation of the apparatus illustrated in FIG- URES 1-5 and theapparatus illustrated in FIGURES 6 and 7 are similar in many respects.In each case, a fluid composition comprising a polymerizable organicmonomer is fed to the inlet end of the casting apparatus and ispolymerized while being carried between the moving belts which aresubstantially parallel for a large portion of the time during which theyare in contact with the composition being polymerized and cast. Thesebelts have substantially no relative motion during the time they are incontact with the composition being polymerized and cast and are inclinedat an angle to the horizontal. When the path of the moving belts issubstantially a straight line as illustrated in FIGURE 6, this angle ofinclination will depend upon the viscosity of the composition beingpolymerized, the polymerization conditions, the length of the belts andother factors. However, given the principles of the present invention,only routine experimentation is required to determine that angle ofinclination which will be sufficient to maintain the desired spacingbetween the moving belts by means of the uid pressure being exerted bythe material being polymerized. In general, it has been found that thisangle may vary from about 0.5 to about 5.0.

When the embodiment illustrated in FIGURES la and 1b is used, theconfiguration of the curved path defined by the lower belt will be thenatural sag or natural curvature of these belts. As used herein, naturalcurvature means that curvature which a given belt would assume when theends thereof are supported at a given height under a given tension. Ithas been found preferable to calculate the natural curvature for a giventension and to arrange support rollers 37 and 38 such that they assumethe path of this natural curvature and then to hold the upper belt undergreater tension than the given tension and the lower belt under lesstension than the given tension such that the tension on the upper belttends to` pull the lower run thereof against rollers 37 and the upperrun of the lower belt can relax against rollers 38. Thus, in a preferredembodiment, the stainless steel belts which are held under a tension ofabout 5100 p.s.i. for the lower belt and 7000 p.s.i. for the upper belt,these being somewhat less than maximum tension, maximum tension beingdefined as the greatest tension to which the belts may be subjectedwithout reaching the yield point. The concept of natural curvature andthe methods of calculating this curvature are known to those skilled inthe art and, as such, do not form a part of the present invention. Forexample, these considerations are discussed in The Sandvik Steel BandConveyor Handbook published in 1956 by The Sandvik Steel Works Co. Ltd.,which is incorporated by reference herein.

In a preferred embodiment, the curved incline of the belts extends overonly a portion of their path between the rollers. Thus, as shown inFIGURE 1b, the path of the belts is substantially horizontal from apoint shortly before they enter the postcuring unit housing 30 untilthey reach rollers 14 and 17 and the curved path extends between thelast section of the housing 29 and rollers 13 and 16. The curved portionof this path is tangent to the horizontal portion and this relationshiptaken with the length of the path between rollers 13 and 16 and thehorizontal portion plus the natural curvature of the belts under a giventension will determine the vertical distance above rollers 14 and 17 towhich rollers 13 and 16 must be raised.

It is to be understood that the word inclined as used in the presentspecification and claims is intended to include straight, curved,angular or other paths which may be defined by a belt moving between arst roller and a second roller wherein the Vfirst roller is at a heightgreater than that of the second roller.

The compositions which may be cast according to the present inventionmay be various types of liquids including monomeric compounds in theliquid state, preferably when at normal atmospheric pressure, partiallypolymerized monomeric compounds and solutions of soluble polymericresins in monomeric compounds. In addition, pastes may be employedincluding mixtures of liquid monomeric and solid polymeric organiccompounds which are at least partially soluble in the monomericcompounds.

Monomeric compounds which are in the liquid state at normal atmosphericpressure may contain one CH2=C group per molecule, eg., methylrnethacrylate, butyl methacrylate, styrene and vinyl acetate, or morethan one CH2=C group per molecule, eg., glycol dimethacrylate, allylmethacrylate, diallyl oxalate, etc. Monomeric compounds which containmore than one CII2=C group per molecule when polymerized alone, or inthe presence of a compound containing more than one CH2=C group permolecule and which is not an overwhelmingly major component of themixture, e.g., not more than about first form viscous liquids, then gelswhich have only a small solubility in monomers and then insoluble solidpolymeric resins. Thus, not all partially polymerized or fullypolymerized monomeric compounds are suitable for use in the presentinvention particularly when the monomers contain more than one CH2=Cgroup per molecule.

The liquids and pastes used in the present invention may `containplasticizers such as tricresyl phosphate or dibutyl phthalate; fillerssuch as iinely divided inorganic materials; reinforcing materials suchas fabrics, glass fibers, wire, etc.; dyes and pigments, providing thatthese materials do not prevent the polymerization of the monomericcompounds contained in these mixtures. The monomeric material may alsocontain a parting agent to facilitate separation of the belts from thecast product and polymerization catalyst such as benzoyl peroxide oracetyl peroxide.

Patterned or contoured surfaces may be produced on the cast sheet byproviding a matrix having a surface which is the negative of thatdesired on the surface of the cast sheet between one or both of themoving belts and the composition being cast. This matrix may be attachedto the surface of the moving belt or may be mounted on separate rollersoperated at such a speed so as to move the matrix material atsubstantially the same linear speed as that ofthe moving belts.

The present invention is further illustrated by the following examplesin which all proportions are by weight.

Example I A viscous syrup comprising 98% methyl methacrylate, 0.5%laurcyl peroxide (catalyst), .000l% Zelec (parting agent), 0.5% ethyleneglycol dimethacrylate and 0.05% Tinuvin P (ultraviolet absorber) is fedto the inlet end of an apparatus of the type illustrated in FIGURES 1aand lb. The length of the curved portion of the path of these belts isapproximately 135 feet while the length of the horizontal portion ofthese belts is approximately 30 feet. The vertical distance between thetop and bottom of the curved portion is 6 feet. The belts are 54 incheswide. The temperature in housing 29 is maintained at 160 F. by hot waterspray means of the type illustrated in FTGURES 2 and 3 and 'thetemperature in housing 30 is maintained at 225 F. by electric heatingmeans to postcure the cast sheet. The speed of the belts is 3 feet perminute. The belts are set 1A; inch apart to produce a cast sheet havinga thickness of approximately Ms inch. These sheets are transparent, haveextremely smooth surfaces, are free from internal voids or bubbles andare substantially free from internal strain.

The belts used were fabricated from `highly polished stainless steel.

Example -II the upper belt in such a manner as to maintain a constantspacing of 1/s inch between the belt. The monomeric material was firstexposed to a temperature of 150 F. in the water flooding zone to curethe monomeric syrup and was then subjected to a temperature of 250 F. inthe hot air oven to postcure the cast sheet.

It is to be expected that certain variations of the specific embodimentsdisclosed herein will `be devised by those skilled in the art. Thus, thepresent invention is of the scope of the appended claims.

We claim:

1. A process for preparing substantially rigid sheets and the like whichcomprises passing a tiuid composition comprising a polymerizable organicmonomer between and in contact with two moving surfaces, providing apair of seals between said surfaces such that said uid composition isconfined in a space bounded on -two oppoing sides by said seals,allowing said uid to lill the cavity defined by said surfaces and saidseals, passing said composition and moving surfaces through at least onezone in which said composition is subjected to conditions which causesaid monomer to polymerize, one of said surfaces being disposed abovethe other of said surfaces in said zone, said upper surface being ofsuch a size and weight that it would bow downwardly into said uid andtoward said second surface if said surfaces were 8 horizontal in saidzone, maintaining the path of said surfaces through said zone atanincline to the horizontal to cause a uid head to be developed by saidfluid composition, said incline being sufficient for the fluid -pressure between said surfaces to maintain said surfaces substantiallyparallel to each other and the spacing between said surfaces at apredetermined distance, bringing said surfaces together to an extentsufficient to compensate for i any shrinkage of the composition duringpolymerization such that said surfaces are maintained in contact withsaid lcomposition during polymerization, and removing the resultantpolymerized sheet from both of said surfaces.

2. The process of claim 1 wherein said incline comprises a substantiallystraight line.

3. The process of claim 1 in which said monomer contains at least oneCH2==C group.

4. The process of claim` 1 in which the moving surfaces are metal andthe monomer is polymerized by heat.

5. The process of claim 4 in which said moving surfaces com-prisestainless steel.

6. The process of claim 1 in which said surfaces are heated by sprayinghot liquid thereon.

7. The process of claim 1 wherein said surfaces are heated by applyinghot liquid thereto.

8. The process of claim 1 wherein said polymerized` sheet is postcuredby passing said sheet :through a postcuring oven before said sheet isremoved from said surfaces.

9. The process of claim 1 wherein said fluid composition comprises anacrylic monomer.

10. A process for preparing substantially rigid` sheets and the likewhich comprises passing a fluid composition comprising a polymerizableorganic monomer between and in contact with two moving surfaces,providing a pair of seals between said surfaces such that said fluidcomposition is confined in a space bounded on two opposing sidesby saidseals, allowing said uid to fill the cavity de-` fined by said surfacesand said seals, passing said moving surfaces and said compositionthrough a first heating zone, heating said monomer to an extentsufficient to cause polymerization thereof in said, first heating zone,one of said surfaces being disposed above the other of said surfaces insaid zone, said `upper surface being of such a size and weight that itwould bow downwardly into said uid and toward said second surface ifsaid surfaces were horizontal in said zone, `maintaining the path ofsaid surfaces through said first zone `being at an incline to thehorizontal to cause a fluid head to be developed by said fluidcomposition, said incline being sufficient for the fluid pressureexerted by said fluid composition between said surfaces to maintain saidsurfaces substantially parallel to each other and the spacing betweensaid sur-` faces at a predeterrnineddistance, passing said movingsurfaces and composition through a second heating zone,

subjecting `said polymerized monomer ito heat to cause postcuringthereof in said second heating zone, bringing said moving surfacescloser together as they pass through said rst heating zone wherebyshrinkage of said composition during polymerization is compensated forand said composition `is maintained in contact with both of saidsurfaces until polymerization and postcuring have been completed, andremoving the resultant polymer-ized sheet from both of saidy surfaces.

References Cited UNITED STATES PATENTS ll/l960 `Hunt z 18--6 3/1950Williams 264-22 ROBERT F. WHTTE, Primary Examiner.

1. A PROCESS FOR PREPARING SUBSTANTIALLY RIGID SHEETS AND THE LIKECOMPRISES PASSING A FLUID COMPOSITION COMPRISING A POLYMERIZABLE ORGANICMONOMER BETWEEN AND IN CONTACT WITH TWO MOVING SURFACES, PROVIDING APAIR OF SEALS BETWEEN SAID SURFACES SUCH THAT SAID FLUID COMPOSITION INCONFINED IN A SPACE BOUNDED ON TWO OPPOING SIDES BY SAID SEALS,ASLLOWING SAID FLUID TO FILL THE CAVITY DEFINED BY SAID SURFACES ANDSAID SEALS, PASSING SAID COMPOSITION AND MOVING SURFACES THROUGH ATLEAST ONE ZONE IN WHICH SAID COMPOSITION IS SUBJECTED TO CONDITIONSWHICH CAUSE SAID MONOMER TO POLYMERIZE, ONE OF SAID SURFACES BEINGDISPOSED ABOVE THE OTHER OF SAID SURFACES IN SAID ZONE, SAID UPPERSURFACE BEING OF SUCH A SIZE AND WEIGHT THAT IT WOULD BOW DOWNWARDLYINTO SAID FLUID AND TOWARD SAID SECOND SURFACE IF SAID SURFACES WEREHORIZONTAL IN SAID ZONE, MAINTINING THE PATH OF SAID SURFACES THROUGHSAID ZONE AT AN INCLINE TO THE HORIZONTAL TO CAUSE A FLUID HEAD TO BEDEVELOPED BY SAID FLUID COMPOSITION, SAID INCLINE BEING SUFFICIENT FORTHE FLUID PRESSURE BETWEEN SAID SURFACES TO MAINTAIN SAID SURFACESSUBSTANTIALLY PARALLEL TO EACH OTHER AND THE SPACING BETWEEN SAIDSURFACES AT A PREDETERMINED DISTANCE, BRINGING SAID SURFACES TOGETHER TOAN EXTENT SUFFICIENT TO COMPENSATE FOR ANY SHRINKAGE OF THE COMPOSITIONDURING POLYMERIZATION SUCH THAT SAID SURFACES ARE MAINTAINED IN CONTACTWITH SAID COMPOSITION DURING POLYMERIZATION, AND REMOVING THE RESULTANTPOLYMERIZED SHEET FROM BOTH OF SAID SURFACES.